A study links African clawed frogs, which were brought to California decades ago for use in pregnancy tests, with a deadly pathogen.

African clawed frogs were first brought to California decades ago to help doctors figure out whether their patients were pregnant. After new technology made those pregnancy tests obsolete, the creatures were let loose, and thrived for decades in the state's drainage ditches and ponds.

Now there are signs that the proliferation of African clawed frogs may be putting native species in peril. A study published last week in the journal PLOS ONE reveals that they carry a deadly fungus responsible for wiping out vast numbers of amphibians around the world.

The spread of the deadly Batrachochytrium dendrobatidis fungus is contributing to one of the greatest disease-caused losses of biodiversity in recorded history. The fungus causes amphibians' skin to harden, interfering with the regulation of electrolytes and eventually causing cardiac arrest.

Entire species of amphibians, which are among the oldest vertebrates on Earth, have been driven to extinction since the fungus first was described in 1999.

"This pathogen is like no other pathogen we have seen," said Vance Vredenburg, a biologist at San Francisco State University who led the new study.

African clawed frogs don't appear to succumb to the fungus' deadly effects. But its presence in a hardy and thriving population that is difficult to control has raised alarm among conservationists.

Native to an arid region of South Africa, the Xenopus laevis species is extremely hardy. The frogs can adapt to cold and drought and can survive for several years burrowed in mud. Efforts to eradicate them here and elsewhere have proved fruitless.

"Once they're established in the wild they're very difficult to eradicate," said Stanford University veterinarian Sherril Green. "I don't know of any place in the world where they have been introduced that has had success in eradicating them."

The fungus has been circulating among African clawed frogs for some time — a sample from 1938, from Africa, tested positive for it. But the fungus had never been detected in California's population. The researchers found signs of it in specimens preserved in a Stanford collection at the California Academy of Sciences in San Francisco's Golden Gate Park. The frogs had been caught in 2001 and 2003, in San Diego and San Francisco counties.

Scientists can't be certain that the African frogs of California are responsible for spreading the fungus to native species, such as the mountain yellow-legged frog. The American bullfrog, a non-native species introduced here during the Gold Rush, also is a known carrier of the fungus. Which species infected the others remains unresolved.

Still, discovery of the fungus among the African frog population in California "tightens the noose around the idea that humans really are responsible for moving this pathogen around," Vredenburg said.

Because of its biological similarity to humans, the species is important to biologists. The frogs' hormone system is analogous to that of humans, and they ovulate when exposed to urine from a pregnant woman. That made them useful for detecting pregnancies — a practice that became widespread in the 1940s and continued until more sophisticated blood tests were introduced in the 1970s.

That's when many hospitals simply released their frogs.

"We weren't as conservation-aware in those days," Green said.

The mystery of the fungus has vexed biologists, who consider it a potential harbinger of emerging diseases that could affect domesticated species crucial for food, such as cattle and poultry.

"Science thrives on predictability," Vredenburg said. "We have very little predictability in this. In the very same habitat, sharing the same lakes and streams, you have frogs that are surviving. They've been infected and are doing fine. For every rule that we put down we find an exception to it."

Even within the same species, some frog populations appear to survive the fungus, Vredenburg said. A South African researcher found that the fungus was not causing mortality in the wild frogs that carried the pathogen. The fungus and frog populations there could be evolving to adapt, a common evolutionary path for pathogens and their hosts.

The mechanics of such a process could be crucial to solving future pandemics.

"We really need to understand the fundamental biology here," Vredenburg said. "That same biology is occurring in other kinds of pathogens."